1. Rooting powder / gel will all work.
2. See the shadows on your white paper? Means it aint working.
3. Your plants have ONE BUD SITE. The main stem tip. Either add more lights now, or prep for ONE BUD per plant.

Veg is where you prep her for flower. Adding more light during flower only will result in a nicer one bud. Get some single CFL on her sides...like 3 of them. This will help develope secondaries of which you show not a single one.

Without them, you will have only one bud at top...may a cuple smaller ones next to stem but thats it.
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Yes you can use other ferts in addition to the Take Root stuff. They are completley different in nature.

However, dont feed nutes if the root system is not developed enuff to utilize the nutes. You can overwhelm the plant and burn her. Right now, plants this small at this long age, do not need nutes added....continue to feed the root system and then in a week, they should be much larger and more mature and capable of handling light nutes.